1. Field of the Invention
This invention relates generally to television receivers and more particularly relates to a television receiver having a plurality of cathode ray tubes, in which the beam current is detected by detecting the cathode current of each cathode ray tube and the brightness of the luminance level is limited on the basis of the detected current.
2. Description of the Prior Art
In the prior art, there has been proposed an ABL (automatic brightness limiting) circuit for protecting a cathode ray tube from heat, for preventing X-rays from being emitted by excessive beam current and for preventing the high voltage generating circuit from being overloaded. That is, a beam current of the cathode ray tube is detected and the detected current is negatively fed back to a brightness adjusting circuit to thereby limit the beam current of the cathode ray tube. Such ABL circuit is similarly provided in a three-tube type video projector which includes cathode ray tubes of, for example, red, green and blue colors.
The ABL controls are classified as average value ABL control in which the ABL control is carried out on the basis of an average value of the beam current and peak value ABL control in which the ABL control is carried out on the basis of a peak value of the beam current.
In the afore-noted video projector of, for example, the three-tube type, the average value ABL control is carried out such that as shown in FIG. 1, the sum of the high voltage currents flowing through cathode ray tubes 1R, 1G and 1B for red, green and blue colors is detected with a detecting circuit 2 and the ABL control is carried out on the basis of such detected current. In FIG. 1, reference letter HV represents a high voltage and reference numerals 3R, 3G and 3B respectively designate drive circuits for the cathode ray tubes 1R, 1G and 1B.
In this case, however, it is difficult to assure that the cathode ray tube demonstrates excellent light emission. For example, let it be considered that an average current of only 1 mA flows in the respective cathode ray tubes 1R, 1G and 1B. In this case, if the brightness of luminance level, namely, the beam current is limited by the detection current, 1 mA, in the case of white, red, green and blue color picture screens, the average beam current flowing through the respective cathode ray tubes 1R, 1G and 1B is limited to the level as shown in the table of FIG. 2A. As a result, it is possible to avoid destroying the cathode ray tubes. However, in the case of the white color picture screen, each average beam current flowing through the respective cathode ray tubes 1R, 1G and 1B is considerably less than 1 mA so that the light emission capability of the respective cathode ray tubes 1R, 1G and 1B will not be obtained. On the other hand, if the luminance level, namely, the beam current is limited by, for example, a detection current 2.2 mA, in the case of white, red, green and blue color picture screens, the average beam current flowing through the respective cathode ray tubes 1R, 1G and 1B will be limited by the level as shown in the table of FIG. 2B so that the brightness in the case of a white picture screen becomes 2.2 times as high as the above case and hence the light emission capability of the cathode ray tube can be obtained. However, in this latter case, for the red, green and blue picture screens, the average current of up to 2.2 mA flows through the respective cathode ray tubes 1R, 1G and 1B, resulting in the disadvantage that the cathode ray tube can be destroyed.
Therefore, in order to demonstrate the light-emission capability of the cathode ray tubes 1R, 1G and 1B as much as possible while safety is maintained, it may be considered that each cathode current is detected so as to thereby detect the beam current and the luminance level is limited as a function of the detected current.
FIG. 3 is a diagram showing the principle. In FIG. 3, reference numeral 4 designates a terminal to which a color video signal SV is supplied. The video signal SV applied to the terminal 4 is supplied to a signal processing circuit 5 which then produces at its output terminals red, green and blue primary color signals R, G and B, respectively. The respective primary color signals R, G and B are supplied through drive circuits 3R, 3G and 3B to the cathodes of the cathode ray tubes 1R, 1G and 1B. A high voltage HV from a flyback transformer 6 is supplied to each anode of the cathode ray tubes 1R, 1G and 1B. Current detecting circuits 7R, 7G and 7B are, respectively, connected to the cathodes of the cathode ray tubes 1R, 1G and 1B. Detected signals SR, SG and SB of the cathode currents that are produced by detecting circuits 7R, 7G and 7B are, respectively, supplied through diodes 8R, 8G and 8B to one input terminal of a comparator 9. In other words, the maximum one of the detected signals SR, SG and SB is supplied thereto. Comparator 9 is supplied at its other input terminal with a reference level signal V.sub.REF. A comparison error signal SC is supplied from comparator 9 to the signal processing circuit 5, in which on the basis of the comparison error signal SC, the luminance level, namely, the levels of the primary color signals R, G and B are limited so as to limit the beam current. In this example, let it be considered that a beam current of only 1 mA is supplied to the respective cathode ray tubes 1R, 1G and 1B. In this case, if the luminance level, and, the beam current is limited by the detected current of 1 mA, in the case of the white, red, green and blue color picture screens, the currents flowing through the respective cathode ray tubes 1R, 1G and 1B will be limited to levels as shown in the table of FIG. 4. Accordingly, the cathode ray tubes will not be destroyed. Moreover, in the case of the white color picture screen, the light-emission luminance of each of the cathode ray tubes 1R, 1G and 1B becomes twice that of the example shown in FIG. 1. As mentioned above, if the circuit arrangement is as shown in FIG. 3, it is possible to make each of the cathode ray tubes 1R, 1G and 1B demonstrate its light-emission capability to the maximum while safety is maintained.
However, in a television receiver in which the beam current is detected by detecting the cathode current, due to parasitic capacity (for example, 8 pF to 15 pF) which is produced around the cathode, the leads and the like, a charging current that charges the parasitic capacity is included in the cathode current so that the detected current exceeds the beam current. As a result, it is impossible to detect the beam current correctly and hence there is a disadvantage that the correct ABL operation can not be accomplished. More particularly, in FIG. 5, if reference numeral 10 is the parasitic capacity, the parasitic capacity 10 is charged with a current Ic and the cathode detected current is obtained by adding the current Ic to a beam current Ib. In FIG. 5, reference numeral 1 designates a cathode ray tube, 3 a drive circuit and 7 a current detecting circuit.
Further, while in the above three-tube type video projector, the peak values of the red, green and blue primary color signals R, G and B that are respectively supplied to the cathode ray tubes 1R, 1G and 1B are detected and on the basis of the detected peak values the ABL control is carried out. When the set value for the ABL is made constant, if the characteristic of the cathode ray tubes 1R, 1G and 1B such as mutual conductance, cutoff frequency, gamma correction and so on are scattered, the beam current which will be limited is scattered so that accurate ABL control can not be obtained.